Drop incoming fe80:: packets silently now.

IPv6 has the concept of "link local" addresses, fe80::<host id>, which
normally are present on every link, and are used for stuff like DHCPv6,
neighbor discovery, etc.

OpenVPN point-to-multipoint mode currently does neither configure them on
tun interfaces, nor are they handled in a meaningful way if a client OS
always has them (like Windows or Solaris) - so the log fills with many
lines of "MULTI: bad source address from client [fe80::...]", serving
no useful purpose.

This patch just recognizes IPv6 LL packets and silently drops them.

Further patches can build on this and add full link-local support, which
would require address learning (as the addresse are based on host IDs, not
assigned by the server).

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1402080235-24409-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8773
(cherry picked from commit 70f1864188ad00451683cabf51e56b7730250c40)